Influence of Headgroups in Ethylene-Tetrafluoroethylene-Based Radiation-Grafted Anion Exchange Membranes for CO2 Electrolysis

The performance of zero-gap CO2 electrolysis (CO2E) is significantly influenced by the membrane’s chemical structure and physical properties due to its effects on the local reaction environment and water/ion transport. Radiation-grafted anion-exchange membranes (RG-AEM) have demonstrated high ionic conductivity and durability, making them a promising alternative for CO2E. These membranes were fabricated using two different thicknesses of ethylene-tetrafluoroethylene polymer substrates (25 and 50 μm) and three different headgroup chemistries: benzyl-trimethylammonium, benzyl-N-methylpyrrolidinium, and benzyl-N-methylpiperidinium (MPIP). Our membrane characterization and testing in zero-gap cells over Ag electrocatalysts under commercially relevant conditions showed correlations between the water uptake, ionic conductivity, hydration, and cationic-head groups with the CO2E efficiency. The thinner 25 μm-based AEM with the MPIP-headgroup (ion-exchange capacities of 2.1 ± 0.1 mmol g–1) provided balanced in situ test characteristics with lower cell potentials, high CO selectivity, reduced liquid product crossover, and enhanced water management while maintaining stable operation compared to the commercial AEMs. The CO2 electrolyzer with an MPIP-AEM operated for over 200 h at 150 mA cm–2 with CO selectivities up to 80% and low cell potentials (around 3.1 V) while also demonstrating high conductivities and chemical stability during performance at elevated temperatures (above 60 °C).

Chemical structures of different AEMs including the ETFE-based radiation-grafted AEMs that were the main subject of the paper.
Commercial AEMs Selemion ® AMV, Fumasep ® FAA-3-50, and PiperION) were purchased from FuelCellStore, while Sustainion ® X37-50 RT was purchased from Dioxide Materials. A porous silver membrane with a pore size of 1.2 μm (99.9 % purity) was obtained from Sterterlich Inc. and used as the cathode. The commercial IrO2-coated carbon paper anode was purchased from Dioxide Materials. KHCO3 (Sigma-Aldrich 99.995 % trace metal basis) and KOH (Sigma-Aldrich 99.95 % trace metal basis) were used as an electrolyte for cell testing or membrane activation solutions.

Methodology for the RG-AEMs synthesis
The RG-AEMs were synthesized via the radiation-grafting peroxidation method. ETFE was selected as a substrate rather than LDPE and HDPE, as quick screening CO2E cell tests showed that ETFE-based RG-AEMs led to the least parasitic H2 generation.

Grafting step:
The electron-beamed ETFE films with approximate area of 12 × 12 cm 2 were submerged in an aqueous grafting solution containing 5% vol vinylbenzyl chloride (VBC, 97% purity, mixture of 3-and 4-iomers, 700-1100 ppm nitromethane or 50-100 ppm 4-tertbutylcatechol inhibitors, purchased from Sigma Aldrich) and 1 %vol 1-octyl-2-pyrrolidone in deionised water. The grafting solution was then purged with N2 for 1 h before sealing the vessel and heating it to 70 °C for 24 h. Post-grafting the ETFE-g-poly(vinylbenzyl chloride) grafted films [designated ETFE-g-p(VBC) in the main paper] were thoroughly washed in acetone and toluene to remove any excess VBC as well as any non-grafted poly(VBC) homopolymer that may have formed during the graft reaction. The ETFE-g-p(VBC) membranes were then dried at 50 °C under reduced pressure (vacuum oven) for 3 h to remove all traces of solvent. The degree of grafting (dog, %) of the ETFE-g-p(VBC) intermediate membranes was calculated using the following equation: where mg is the mass of the ETFE-g-p(VBC) membrane and mi is the mass of the initial irradiated ETFE film. The dog for the ETFE-g-p(VBC) made from E25 and E50 were 80 % and 68 %, respectively. It was clear that when using the same grafting process (and radiation dose), the thicker ETFE grafted to a lower degree. However, we cannot use higher radiation doses with ETFE as this would lead to RG-AEMs that are too mechanically weak for use in electrochemical cells (especially at elevated temperatures), 6 as the electron-beam treatment process breaks a proportion of the C-C bonds in the ETFE chains (alongside the desired ability to graft monomers).

Figure S2
Box plots containing extracted integrated Raman peak area data for spectra recorded on 30 random spots (ca. 2 μm diameter laser spot sizes and depths, laser λ = 785 nm) recorded on both sides of the ETFE-g-p(VBC) made from both 25 and 50 μm thick ETFE substrate films (E25 and E50). The peak at 1612 cm -1 derives from the grafted poly(vinylbenzyl chloride) chains, while the peak at 835 cm -1 derives from the ETFE substrate films. Plots show max (top bar), interquartile range (box), median (middle bar), and minimum (bottom bar). The means and sample standard deviations (and relative standard deviations, RSD) are given above the boxes. This data is to evaluate grafting homogeneity (and levels -higher mean peak area ratios for a higher dog). An RSD of 10 % or lower is normal for such lab-fabricated RG-AEMs.

Table S2
The reaction conditions used to aminate the AEMs in this study (of both thicknesses).  Table S2. Postamination all RG-AEMs were thoroughly washed in ultrapure water (UPW) before subsequent heating in fresh UPW at 60 °C for 18 h. This is to ensure that any unreacted amine is removed from the membrane.

Amination reaction conditions used
Ion-Exchange process to ensure Cl − forms for storage and initial experiments: To ensure complete conversion to the pristine Cl − anion form RG-AEMs, the crude, as synthesised, RG-AEMs were submerged in aqueous NaCl solution (1 mol dm −3 ) for 24 h with the NaCl solution being refreshed three times during this period. The resulting AEMs were then removed and S6 thoroughly washed with UPW to remove any excess free-ions: both co-ions (Na + ) and excess counter-ions (any Cl − that are not charge balancing the quaternary ammonium positive charges). The pristine Cl − anion form RG-AEMs were stored under UPW until use.

RG-AEM characterization
Raman Spectroscopy Raman spectra were recorded on dry samples of the ETFE-g-p(VBC) and final RG-AEMs using a Renishaw InVia Reflex Raman Microscope equipped with a 785 nm IR laser and a 20× (NA = 0.40) objective. All Raman data was collected and baseline corrected using Renishaw WiRE Software (Renishaw PLC, UK), with normalization and integration of band intensities conducted using Spectragryph (Spectroscopy Ninja, Germany).

Ion-exchange capacities (IEC)
The ion exchange capacities (IEC) were determined using potentiometric AgCl precipitation titrations. For each RG-AEM in the Cl − form, a dehydrated sample of known dry mass (mdry) was immersed in 25 mL aqueous NaNO3 solution (1.2 M) and continuously stirred for 16 h. Subsequently, the solution (still containing the RG-AEM sample) was acidified with aqueous 2 mL HNO3 (2 M) and titrated against aqueous AgNO3 standard solution (0.02000 ± 0.00006 M). A Metrohm 848 Titrino Plus autotitrator equipped with an Ag/AgCl Titrode was used for the dynamic equivalence point titrations (DET). The endpoint was calculated as the peak maxima in the first differential plot of potential vs. titrant volume data. IEC was calculated with Equation S2 .
where Ep represents the endpoint volume, Cst is the AgNO3 standard concentration solution, and mdry is the mass of the dry RG-AEM(Cl⁻) sample under analysis. This procedure was undertaken on n = 3 samples of each RG-AEM.
Water uptake (WU) and through-plane swelling (TPS) A RG-AEM(Cl⁻) sample was removed from UPW storage and the excess surface water was removed by blotting with a filter paper. The hydrated mass (mhy) and thickness (Thyd) were then recorded immediately. Masses were recorded on a 4 decimal place (0.1 mg) analytical balance, and thicknesses were recorded using an outside digital micrometer (precision of ± 2 μm). The RG-AEM(Cl − ) sample was dried under reduced pressure at 50 °C (vacuum oven) for 18 h before the dehydrated mass (mdry) and thickness (Tdry) were recorded. All measurements were conducted on n = 3 samples of each RG-AEM(Cl − ). The gravimetric water uptake, through-plane swelling (TPS), and the hydration number (λ) for each sample were calculated using Equations S3 -S5.
where Mwater corresponds to the molecular mass of water (18.015 g mol -1 ). Area swelling values were calculated in the same way as TPS values but using the hydrated and dry areas measured at the same time as the thicknesses.
In-plane ion-conductivity The in-plane Cl⁻ and HCO3⁻ anion conductivities of fully hydrated RG-AEM samples between room temperature and 80 °C were measured using a Solartron 1260/1287 combination controlled by ZPlot/ZView software (Scribner Associates, USA). Impedance spectra were collected over a frequency range of 1.0 -10 6 Hz (10 mV a.c. amplitude) with the samples mounted in a 4-probe BekkTech BT-112 test cell (Alvatek, UK). Test cells containing samples of the Cl⁻ and HCO3⁻ RG-AEM forms were then submerged in UPW. The ionic resistances values, taken as low-frequency x-(real)-axis intercepts in the collected Nyquist plots, were used to calculate the conductivities using Equation S6: where L corresponds to the working electrode distances (0.425 cm), and w and T are the width and thickness of the RG-AEM samples, respectively.  1.4 ± 0.01

Electrode Preparation
Preparation of Cu electrodes for CO2E to C2+ products: Cu-based electrocatalysts were synthesized using physical-vapor deposition (PVD). Layers of 150 nm thick Cu (6N grade) were deposited onto commercial gas diffusion layers (Sigracet 39BB purchased from FuelCellStore) by magnetron sputtering (AJA International) in a vacuum environment (10 -6 Torr) at a deposition rate of 1 Å s -1 under 10 sccm Ar with a sputtering pressure of 2 mTorr. Our Cu-electrodes don't require the addition of any ionomers as binder, so we can evaluate the effects of the membrane's chemistry in the AEM/cathode interface and the reaction, without the potential influence of the binder´s chemistry Catalyst characterization: Scanning electron microscopy (SEM) of porous Ag and Cu-GDE catalysts was performed using FEI Quanta 200 FEG instrument with an accelerating voltage of 15 kV in secondary electron mode. In addition, X-ray photoelectron spectroscopy (XPS) measurements were carried out in a ThetaProbe instrument (Thermo Fisher Scientific) with monochromatic Al Kα radiation (1486.7 eV) equipped with a hemispherical analyzer. Scans were made in the binding energy range of 0−1400 eV with an analyzer pass energy of 100 eV.

Cell configuration and electrochemical tests:
All electrochemical experiments were performed on a commercial electrolysis cell (Dioxide Materials) in a zero-gap MEA configuration. The assembly consisted of loading a fresh AEM (area = 7.4 cm 2 ) inserted between a cathode (area = 2.25 cm 2 ) and anode (area = 4 cm 2 ). PTFE gaskets further sandwiched the MEA device, which helped prevent electrolyte leakage and potential shortcircuiting. The system was mechanically pressed, using cell bolts fastened with a torque of 3 N·m to guarantee an uniform and adequate compression. A Bio-Logic VSP 300 potentiostat with a booster channel was used for electrochemical measurements.
The CO2 (AGA, purity 4.5) flow was set using a mass flow controller (MKS Instruments Inc.) and further humidified by sparging it into a container filled with UPW before being fed to the cathode (standard flow= 40 mL min -1 ). A liquid trap was installed on the outlet line from the cathodic gas to prevent water from entering the gas chromatograph (GC). This also allowed for the collection of liquid effluent species. The anode was fed with aqueous 0.1 M KHCO3 and continuously recirculated (40 ml min -1 ) using a diaphragm pump (KNF Neuberger Inc.). The standard conditions for gas flow in this work are 293 K and 1 bar. An Ag/AgCl (3.5 M NaCl internal solution) was employed as the reference electrode. Current interrupt and impedance techniques measured the uncompensated and charge transfer resistances. An illustration of the electrochemical cell and the reaction setup used for these experiments can be found in Figure  S3.
The molar outlet flow of the cathodic and anodic streams was measured using a volumetric flow meter (MesaLabs Defender 530) located downstream of the gas chromatograph (GC). The gas product's composition was quantified with a PerkinElmer Clarus 590 GC equipped with a Molecular Sieve 13x, and HayeSep Q packed column using Ar (10 ml min -1 ) as the carrier gas, and a thermal conductivity detector (TCD). Liquid product analysis was carried out with Agilent Infinity 1260 high-performance liquid chromatography (HPLC), equipped with Aminex HPX-87H column, refractive index (RID), and diode-array (DAD) detectors.   Figure S5 SEM images of the Cu-based catalyst A) gas-diffusion layer (GDL), B) Cu-GDE coated on microporous carbon layers of the commercial SG-39BB GDL before the electrochemical reaction and C) Post-reaction Cu-GDE S12

Calculation for partial current densities and Faradaic efficiencies
Gas and liquid products are quantified using GC, and HPLC analysis techniques. Partial current density and Faradaic efficiency are two of the methods used in this study to estimate the selectivity and electrochemical performance.
Faradaic efficiency can be defined as the amount of electric charge required to form a desired product over the total charge. It represents the selectivity towards a specific product and can be tuned to improve the conversion and reducing the energy consumption. The calculation of the parameter is expressed in Eqn. S7 where z corresponds to the number of electrons required per mol (z = 2 for CO and H2), n is moles of the specific products, F is the Faraday´s constant, and Q is the total charge (current × time).
Furthermore, the current density can be defined as the ratio between the total current and the electrode area (geometric or ECSA). The partial current density is associated to the specific product and the product's reaction rate since the electrons transferred in a chemical reaction is proportional to the reaction's extent:

Calculation of Faradaic efficiency, partial current density and crossover for HCOO⁻
As mentioned in the main paper, the liquid products analysis was conducted using HPLC for the liquid trap at the cathode and the anolyte. The formate crossover across the membrane and further oxidation over the IrO2 anode limited the quantification of the total Faradaic efficiency and the partial current density.
Initially, we measured the liquid products at the anolyte and the cathode (collected from the water trap) and correlated the generation of formate in terms of charge. To illustrate these calculations, we take as reference experiments done with the MPIP-AEM (25 μm) at 150 mA cm -2 ( Figure 3C). Total passed charge (C) = I * t = 1083.24 C Ratio of corresponded charge (%) = q HCOO− q total = 3.68%

S13
While the crossover and oxidation of the formate is presumed to happen fast (as evidenced in Table S5 and comparison between charge ratios), the quantification of the liquid in postreaction techniques doesn't provide a good estimation of this product selectivity and reaction extent. Therefore, we assumed that over Ag-based catalysts, the "unaccounted product" is attributed to the CO2E to HCOO⁻ and therefore calculated using Eqn.S9 and S10 Larrazabal et al. 7 conducted previous experiments to estimate the FE of HCOO⁻ oxidation over IrO2/C at 200 mA cm -2 , showing that the decrease amount of HCOO⁻ (in terms of concentration and remained charge) can be related to a FE for HCOO⁻ ca. 20%.

Figure S6
Partial current densities (ji) for different products as a function of the total current density over Ag-electrocatalyst and 0.1 M KHCO3 anolyte with our different RG-AEMs. Unaccounted products are assumed to be formate. The error bars in such represent the standard error of the mean of three independent measurements.

Neutralization reactions and gas evolution at the anode
Neutralization reactions: Gas evolution at the anode: Combining the neutralization reaction with the gas evolution at the anode, we have:

Water Transport across the AEM
Experimental set-up for these ex-situ measurements of the hydraulic water permeation were based on preliminary studies conducted by Duan et. al 8 and Luo et al. 9 for different ionexchange membranes. By measuring the hydraulic water permeation flux (JHP) in terms of the pressure gradient in the cell, we can correlate the permeability using Equation S11. 10 where k is the permeability (cm 2 ), µ the water viscosity (mPa s), ∆ the pressure gradient (bar), the membrane thickness (µm). We estimate the permeability by calculating the slope of the curves from Figure S10, and such results are reported on Table S6.